1. Field of the Invention
The present invention relates to an image forming apparatus which reduces a ripple or curling of a sheet by applying moisture or an aqueous solution to the sheet having an image that is formed using an electrophotographic system and fixed by a fixing unit.
The invention can be applied to a technique for suppressing a ripple or curling of a sheet by applying moisture or an aqueous solution to the sheet in a fixing unit provided in an electrophotographic system image forming apparatus and the like.
2. Description of the Related Art
In the related art, an image forming apparatus using an electrophotographic system develops a latent image formed on a photosensitive drum as an image bearing member to convert the latent image into a visible image, and transfers the visible image (toner image) onto a sheet using an electrostatic force. Then, the transferred image is thermally fixed so that an image is recorded and formed on the sheet.
In the image forming apparatuses, such as an electrophotographic apparatus and an electrostatic recording apparatus, which form an image using toner, the toner image formed on the sheet is thermally pressed to be fixed. In the related art, as a fixing unit in the image forming apparatus using an electrophotographic system such as a copying machine, there is known a heat-roller fixing unit that fixes the toner image on a sheet. In the heat-roller fixing unit, a heat-roller fixing technique is employed, in which a fixing nip portion is formed by pressing a pressure roller against a fixing roller having a heater embedded therein to perform image fixation.
In the heat-roller fixation type fixing unit, a sheet is introduced into a pressing nip portion (fixing nip portion) obtained by pressing the pressure roller having elasticity and the fixing roller heated by an internal heat source such as a halogen heater to each other to maintain the roller surface at a predetermined temperature, and the sheet is conveyed while the sheet is nipped. As a result, an unfixed toner image is thermally fixed on the sheet surface. In this process, since heat and pressure are applied to the toner and the sheet, moisture inside the sheet is evaporated while the sheet is pressed.
A ripple and curling occur in a paper sheet due to the moisture amount change of the sheet and the stress applied to the sheet during the above-mentioned fixing process. In terms of a fiber level of the paper sheet, paper is a tangle of fibers, and moisture is contained inside or between fibers, so that a hydrogen bonding is generated between the fibers and water.
In the fixing process, if heat and pressure are applied to the paper sheet, a deviation occurs between fibers due to the pressure. If heat is applied to evaporate moisture in this state, a hydrogen bonding is generated between fibers, so that the sheet is deformed. If the sheet is left as it is, moisture is absorbed, and the hydrogen bonding between fibers is separated again depending on the surroundings. However, moisture is not introduced into some fibers, and deformation of the sheet is maintained. The pattern of deformation includes deformation (curling) caused by a difference of expansion and contraction between the front and rear sides of the sheet and deformation caused by a difference of expansion and contraction between the center and the edge of a sheet. Such deformation generates a ripple or curling of a sheet.
A solution for addressing such a problem has been proposed in U.S. Pat. No. 5,850,589, which discloses an apparatus and system used in an electrostatic copy apparatus to prevent the ripple in the edge and curling caused by removing moisture from a paper sheet during the fixing process in the electrostatic copy or print. The proposed apparatus is an apparatus for applying a controlled moisture amount to one or both of surfaces of the sheet, and includes a water-jet which is arranged in both sides of the sheet and has a reservoir for storing a liquid and a pair of pressure rollers having a substantially cylindrical outer surface.
Positions of a pair of pressure rollers are adjusted along their axes to define a nip portion between the cylindrical outer surfaces. This apparatus further includes a control device for controlling moisture applied from the water-jet to a selected part of each passing sheet before the sheet is introduced into the nip portion formed between the cylindrical outer surfaces. As the sheet passes through the fixing unit, the moisture amount in the sheet is reduced to generate a ripple and curling. For this phenomenon, the lost moisture is compensated for by supplementing moisture after the fixing so that a ripple and curling can be alleviated.
The ripple and curling are corrected by compensating for the moisture lost by the heat and pressure during the image fixing process by supplementing moisture to the sheet. As the moisture amount of the sheet increases, a Young's modulus of the sheet itself is reduced, and stiffness is degraded, so that a ripple can be alleviated. It is contemplated that, if moisture is introduced into the gap between fibers, the fixing unit separates the bonding of fibers which was a hydrogen bonding once so that a ripple can be alleviated.
In the technique of U.S. Pat. No. 6,259,887, there are provided a conveying portion that conveys a sheet and a spray device that applies moisture to the sheet by spraying water droplets onto the sheet conveyed by the conveying portion. In addition, there is provided a water amount controlling portion that controls a water feed amount of the water feeder depending on the type of the sheet, an image formed on the sheet, an atmospheric temperature, or humidity. Similar to U.S. Pat. No. 5,850,589, U.S. Pat. No. 6,259,887 has been developed to supplement moisture lost by the heat and pressure during the image fixing process and correct the ripple and curling. Furthermore, in the technique of U.S. Pat. No. 6,259,887, the water feed amount is changed depending on the type of the sheet, the image, and the atmosphere, and a suitable amount of water is provided to each sheet depending on each condition, so that the ripple and curling can be alleviated.
However, according to U.S. Pat. No. 5,850,589, while moisture of the sheet is reduced to generate a ripple and curling as it passes through the fixing unit, the lost moisture is supplemented by applying moisture after the fixing so that the ripple and curling can be alleviated. Unfortunately, even when the ripple and curling are alleviated temporarily by applying moisture to the paper sheet, the ripple or curling of the sheet may problematically increase again if the applied moisture is evaporated to dry the sheet.
In addition, according to U.S. Pat. No. 6,259,887, while the feed amount of the water feeder is adjusted depending on the type of the sheet, the image formed on the sheet, the atmospheric temperature or humidity, feedability or loadability is influenced if the amount of water applied to the sheet is changed. For example, if the spray amount is too much, water droplets may remain on the surface so that a sheet may stick to a guide during conveying, or sheets may stick to each other during loading.
The invention provides an image forming apparatus capable of alleviating the ripple on each sheet by maintaining a suitable amount of moisture on the sheet without degrading feedability or loadability of the sheet.